The present invention relates to a device for light modulation with regularly arranged controllable light-modulating elements, which comprises a birefringent material whose molecule axes are given a certain orientation as they are affected by controllable forces in order to modulate the phase and amplitude of a sufficiently coherent light wave.
The field of application of this invention are light modulator means, e.g. flat spatial light modulators with high image resolution, which are used as display panels in video and TV devices, projectors and similar devices for holographic representations. In combination with an illumination means and an optical system, these panels can be used as holographic displays for the holographic reconstruction of a scene. The holographic display can be both a direct-view display and a projection-type display. The controllable elements can be the pixels of a light modulator.
The present invention shall be seen in conjunction with a preferably real-time or near real-time holographic representation of a video. In this document, the video comprises a multitude of scenes (frames), each of which being individually encoded in the elements of the light modulator means in the form of a hologram. For encoding a hologram, a number of methods are known which take into consideration the properties of the provided light modulator means.
In the description of this invention, the term ‘regularly arranged controllable elements’ refers either to the pixels of a light modulator or to a continuously variable, non-pixelated encoding surface of a light modulator, which is formally divided into discrete regions by the information to be displayed.
A particular type of a holographic display is known from earlier documents filed by the applicant, e.g. from (1) EP 1 563 346 A2, (2) DE 10 2004 063 838 A1 or (3) DE 10 2005 023 743 A1.
There, the hologram computation is executed on the following basis: A three-dimensional scene is divided into a multitude of object points for encoding and holographically reconstructing. During the passage of coherent light through the light modulator, the controllable elements modulate the amplitude and/or phase of the light in order to be able to reconstruct each object point of the scene again. Each object point is encoded e.g. in a certain region of the encoding surface of the light modulator means, and said region then reconstructs it. This region is referred to as the sub-hologram of this object point. The sub-hologram corresponds roughly to a holographically encoded lens function which reconstructs this one object point in its focal point. The object point must be encoded in the form of complex values. The absolute value of the complex values, i.e. the amplitude, is about constant across the entire sub-hologram, and its magnitude depends on the axial distance of the object point to the screen and on the luminous intensity of the object point. The phase distribution of the complex values within the sub-hologram corresponds roughly to the function of a lens whose focal length depends on the axial distance of the object point to the light modulator means or the screen. Outside the sub-hologram, the value ‘0’ has to be written to the light modulator means for this object point. Only those pixels of the light modulator which lie within the sub-hologram contribute to the reconstruction of that single object point. The entire hologram is obtained by adding up the individual sub-holograms.
The holographic reconstruction of the scene is for example generated in co-action with an optical reconstruction system in a reconstruction volume which stretches between the visibility region and the light modulator means. Wave fronts which are emitted by the holograms of the scene which are encoded on the light modulator means are superimposed in the visibility region, and the reconstructed object points can be seen there from an eye position. The reconstruction of the superimposed modulated wave fronts cannot be seen simultaneously by both eyes of an observer. Individual perspective views of the three-dimensional scene are generated for each eye of an observer in a time- or space-division multiplex process, where said views differ in parallax, but are perceived by the brain as a single holographic 3D representation.
For watching the reconstruction of the three-dimensional scene, the observer can either look at a light modulator means on which a hologram of the scene is directly encoded, and which serves as a screen. This is referred to as a direct-view arrangement. Alternatively, the observer can look at a screen onto which either an image or a transform of the hologram values which are encoded on the light modulator means is projected. This is referred to as a projection arrangement. The eye positions of observers are detected in a known manner by a position finder, which is linked to a computing unit by software means.
The computation of a hologram of the scene, which can also be a computer-generated hologram, and whose values are available in a memory unit in the form of a look-up table, provides in the computing unit complex numbers or complex values for each object point, which must be encoded on, i.e. written to the light modulator means. The complex values are encoded in the pixels of a spatial light modulator (SLM) in the case of a pixelated SLM, or in discrete regions of the continuous encoding surface, where said regions are formally defined by the information to be displayed, in the case of a non-pixelated SLM.
Incident coherent light waves can be modulated by SLMs which only modulate the phase of the light waves, or by SLMs which only effect an amplitude modulation. Further, there are also SLMs which execute a combined phase and amplitude modulation. Fix combinations of amplitude and phase values can be written to these SLMs, but not arbitrary complex values.
Conventional solutions for representing complex numbers on a SLM for example use multiple adjacent pixels of the SLM for representing one complex number. This always causes disadvantages. For example, encoding the amplitudes in multiple adjacent pixels typically leads to a low diffraction efficiency of the light modulator means. If only phase values are encoded in multiple adjacent pixels, an additional time-consuming iterative computing process is needed in order to approximate the values to be encoded as much as possible to the real scene.
Other known solutions for representing complex numbers, e.g. as disclosed in document U.S. Pat. No. 5,416,618, take advantage of a combination of multiple light modulators, e.g. two phase-modulating light modulators, or a phase-modulating light modulator and an amplitude-modulating light modulator. One of the disadvantages here is that a very difficult and laborious adjustment process is necessary in order to precisely match the pixel grids of the two light modulators.
Document U.S. Pat. No. 5,719,650 describes a light modulator for controlling the amplitude and phase independent of each other. It comprises two polarisation-rotating elements with one liquid crystal layer each, each of said elements being disposed between two carrier substrate plates. Moreover, the basic electrodes and the grid electrodes are provided separately for each layer. An alignment of the two elements is already performed during the manufacturing process. However, no SLM is commercially available today which includes only one liquid crystal layer to which the complex-valued information of a holographic scene can be written directly and modulated with light.
As is commonly known, one way of realising a light modulator is based on the use of liquid crystals (LC). Liquid crystals are birefringent materials, where the optical axis of the molecules can be controlled to have a desired orientation, e.g. by applying an electric field. In a nematic liquid crystal, the optical axis is the longitudinal axis of a molecule. In a light modulator of such type, the modulation of the incident light depends on the set orientation of the optical axes of the molecules relative to the direction of light passage through the light modulator, and on the polarisation of the light. LC-type light modulators are known which can be used either as amplitude-modulating light modulators or as phase-modulating light modulators.
Document EP 0583114 describes an optically addressable SLM (OASLM). In addition to the liquid crystal layer and the electrodes, it comprises a photoconductive layer. The conductivity of the photoconductive layer is varied depending on the intensity of the write light which falls on the SLM. If an electric field is then applied through the electrodes and photoconductive layer, the photoconductive layer will affect the field which is applied to the liquid crystal layer depending on its conductivity which is controlled by the write light. The orientation of the molecules of the liquid crystal layer is effected according to the applied electric field, and it then serves to modulate the sufficiently coherent read light. While in an EASLM an individual control voltage must be addressed to each pixel, in an OASLM a constant control voltage is supplied and the local orientation of the molecules is effected by the write light. A conventional OASLM can nevertheless only be used to modulate either the phase or the amplitude of the read light.
In a single light modulator with a single liquid crystal layer, phase and amplitude can therefore not be modulated independent of each other, because an electric field or similar effective force applied to the LC layer can always only modify one parameter in order to affect the orientation of the axes of the liquid crystal molecules. This will be explained in detail below with reference to FIG. 1 for a phase modulation and FIG. 2 for an amplitude modulation.
The drawings of FIG. 1 show schematically a pixelated phase-modulating light modulator, representing the prior art, whose function will be explained below with the example of a region of the modulator which has the size of one pixel P. Only the major elements will be shown and described.
The pixel P is shown to have a frame and it comprises a birefringent material, for example a liquid crystal layer LC with molecules M onto which almost coherent light falls. The direction of light incidence is perpendicular to the plane of the drawing, as is indicated by a circle with a cross in the Figures.
In the top view in FIG. 1a, the molecules M are shown in an initial situation in the off state of the phase-modulating light modulator. The incident light is vertically polarised, indicated by a double arrow, and the optical axes of the molecules M are oriented parallel to the incident light.
In FIGS. 1b and 1c, the pixel P is shown schematically in the on state of the phase-modulating light modulator at a medium voltage. The optical axes of the molecules M are turned out of the plane by a certain angle caused by the applied voltage V. If a maximum voltage is applied, the axes will be oriented at right angles to the plane. The liquid crystal layer LC is embedded within two opposing carrier substrates TS, e.g. glass plates, as is shown in FIG. 1c. The molecules M are controlled in their optical properties by applying an electric field between opposing electrodes E1 and E2. The direction of light incidence is indicated by arrows. The applied voltage V does not change the polarisation direction of the light, but the phase of the light is affected by the orientation of the optical axes of the molecules M.
Things are similar with an amplitude-modulating light modulator according to the prior art, for example as used in an in-plane switching (IPS) display. Its function is shown in the drawings of FIG. 2, again with the example of a region with the size of one pixel P. Similar to the drawings in FIG. 1, FIGS. 2a and 2b show a top view of a pixel P, while FIG. 2c is a side view.
The orientation of the optical axes of the molecules M and the polarisation PO of the incident light in the off state of the pixel P in FIG. 2a are the same as those in FIG. 1a. 
However, the electrodes E1 and E2 are arranged differently in FIG. 2a. The applied voltage V is effective from left to right in this arrangement, while there is no representation of the electric field in the side view. This drawing is schematic and very rough. Since the lateral extent of a pixel is typically larger than few micrometers, the transversal electrode is usually sub-divided in order not to have to apply a voltage that is too large. Consequently, multiple electrodes are connected in series per pixel. However, only 2 electrodes are shown here to maintain a certain clarity of the drawing.
FIGS. 2b and 2c show a pixel P in its on state. If a medium voltage V is applied, the optical axes of the molecules are turned in the plane, as shown in FIG. 2b, which is indicated by the slight inclination of the molecules M. In contrast, in FIG. 2c the molecule axes are not turned in the plane of the top view. The polarisation of the incident light is turned starting from the value PO1 by a certain angle to the value PO2 as the optical axes of the molecules M are oriented when a voltage V is applied at a given thickness of the liquid crystal layer LC. The magnitude of the turning angle of the polarisation is twice the angle between the polarisation PO of the incident light and the resultant orientation of the optical axes of the molecules M.
The amplitude of the light can then be modulated after the passage of the light through the liquid crystal layer LC by a polariser (not shown) which is disposed after this layer. For example, with parallel polarisers a maximum amplitude is obtained without a voltage applied, and an amplitude of zero is obtained with a voltage applied and a turn of the optical axis of the molecules by 45° —and consequently a turn in polarisation by 90°. In summary, it can be noted that using a single light modulator, the incident light waves can always only be modulated with one part of the complex value.
If a combination of two SLMs of a liquid-crystal-type are to be realised by a fix attachment of the two already during the manufacturing stage for simultaneous modulation of amplitude and phase, certain criteria must be observed. Since the two SLM in addition to the LC layer also require carrier substrates, which can be glass plates or flexible layers, there is a relatively large distance between the two LC layers. To achieve a correct complex-valued modulation, the two SLMs have to be disposed such that the pixels are fully congruent, so that the light always passes two accordingly assigned pixels. Because of the distance between the two LC layers, which cannot be completely avoided, this condition is no longer fulfilled already for small angles of incident light beams, not to mention when light is transmitted through the SLM panels at oblique angles. But even in the case of a perpendicular incidence of the light, an imprecise adjustment of the light source which is assigned to the two pixels may cause the light to pass two different pixels in the two SLM panels.
This disadvantage particularly applies to small lateral pixel sizes of few micrometers, which are, however, particularly preferred in the context of holographic reconstructions. This is why a correct reconstruction of a holographic scene is difficult to achieve with a combination of two SLMs.